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I recently acquired an older model of PC, and the network card in it did not work. A friend of mine had a spare laying around for me to use as a replacement, but I noticed that it only had 4 of the 8 pins to connect to the cable. He said this wasn't a problem and that it would work, but couldn't explain why (the non-functional one that came with the PC had all 8). I tested it out, and sure enough, it works. How can it work if it only has 4 of the 8 connectors?

Wikipedia states:

Conductors required
10BASE-T and 100BASE-TX Ethernet connections require two cable pairs. 
1000BASE-T Ethernet connections require four cable pairs.

Does it matter which two cable pairs are present? Or can any two pairs be used?

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might want to change the word "connectors" in the question to wires. – Keltari Aug 28 '11 at 0:42
For an answer to your edit, 4, 5 7, 8 are the pins for half duplex, you can look up Ethernet over twisted pair on Wikipedia. I would link it but I'm on my cell phone. – MaQleod Aug 28 '11 at 1:22
up vote 8 down vote accepted

For 10 and 100 base T cable, only 2 pairs of wires are used. Some cable manufacturers save money by not including all the wires. 1000 base T requires all 4 pairs of wires.

Take a look at a standard land based telephone cord. Some have 2 wires, some have 4. Only 2 wires are needed to function, the other 2 are for a second phone line.

Check out the Wikipedia entry on Category 5 cabling for more detail.

To answer your modified question, yes, the pairs do matter. Look at this article. The transimt (tx) and receive (rx) wires are noted in the diagram.

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+1 Thank you, very helpful. However, it raised another question which the Wikipedia article did not answer. Question has been edited. – yoozer8 Aug 28 '11 at 0:31
yes, i amended the answer. – Keltari Aug 28 '11 at 0:37

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